The present invention relates, in general, to the field of short detection circuits and methods for electrical loads. More particularly, the present invention relates to a circuit and method of especial utility in automotive regulators having a field coil driven by a Darlington transistor.
The great majority of modern automobiles utilize an alternator as a power source for the vehicle's electrical system as well as to recharge the starting battery. Such alternators incorporate a stator coil surrounding a rotating field coil mounted within a stamped or cast metal case. In most current applications, this case is bolted to the engine and is therefore at a negative, or ground potential with respect to the field coil which is tied to a supply voltage line at either the positive terminal of the battery or the diode trio of the alternator.
This field coil is ordinarily driven by means of a Darlington transistor, the function of which is controlled by the output of a voltage regulator. Increasingly, the output Darlington and regulator are mounted internally within the alternator case in a module comprising a hybrid integrated circuit or other type of common substrate device where a precise interrelation of temperature coefficients (TC) among the various devices comprising the regulator and Darlington are necessary due to the latter's power dissipation as well as heat internally generated within the alternator or otherwise experienced in the environment of the engine compartment.
A particularly high failure rate for such alternators occurs when the Darlington side of the field coil becomes shorted to the battery. Under ordinary operating conditions, approximately one volt appears across the saturated Darlington resulting in a power dissipation of about 5 watts. When shorted to battery, currents on the order of 10 amps may be experienced at a level of 10 volts or more resulting in a 20 fold increase in power dissipation through the Darlington resulting in its catastrophic failure even when experiencing only a transistory short. Such transitory shorts are most apt to occur due to foreign material within the alternator which may short the field coil brush connection to the supply brush connection. Although manufactured and tested separately, the regulator and alternator are generally assembled together and installed in a new vehicle such that a momentary short may occur when the new vehicle is started up to exit the assembly line. In such instances, the small metal filings or flash from the alternator case or other machined portions of the engine which can precipitate such a short will cause the vehicle to be rejected and necessitate replacement of the alternator/regulator combination. Even though the alternator can later be disassembled and a new regulator installed, the regulator is nonetheless irreparably damaged due to the short circuit current surge in the Darlington driver, however transistory. Were it possible to prevent such overcurrent damage for even a few minutes of operation, generally any internal debris will be blown out of the alternator by its rotation and the above-described failure mode would be much less frequently encountered.
Concomitant concerns with internally mounted alternator regulators include the size of the integrated circuit die and the interdependent number and spacing of bonding pads to the regulator chip module. While die size, of course, directly relates to chip cost, generally spacing between bonding pads must, nonetheless, be a minimum of about 20 mils. Therefore, the number of bonding pads required to interface with the external Darlington and other devices will to a great extent dictate the physical dimension of the die.
Prior art devices have heretofore required two pins for effective alternator field coil operation. One pin is utilized to supply a driving current to the external Darlington base terminal while a second feedback pin is resistively connected to the emitter of the Darlington. An additional resistor couples the feedback pin to circuit ground while a sense resistor similarly connects the emitter of the Darlington to ground. Thus, an additional bonding pad and two trim resistors are necessary to sense voltage across the sense resistor at the feedback pin connected to the Darlington emitter. Moreover, by the nature of the overcurrent sensing employed through the use of a feedback pin, a precision low voltage reference is required within the regulator. Such a precision low voltage reference is inherently different to provide as well as requiring a not inconsiderable amount of on-chip real estate.